This invention relates generally to laminated window panels and more particularly is concerned with an improved electrically conducting laminated aircraft glazing.
The construction and use of electrically heated laminated panels for aircraft windows or windshields is well known. Generally, the panel consists of an electrical heating circuit located between two or more sheets of glass or other rigid transparent plies which are bonded together by a thermoplastic interlayer material such as polyvinyl butyral or silicone resin. Specific embodiments of the basic panel may assume various shapes and the arrangement of the panel components may vary depending upon particular design requirements. Additional components such as temperature control sensing devices may be incorporated into the basic panel structure.
The sheets of glass and/or rigid plastic used in the panel may be of the same or different sizes. The interlayers are usually relatively thick, varying from 1/8 inch to 1/2 inch in thickness, and the peripheral margins thereof may be cut flush with the edges of adjacent rigid sheets or may extend beyond the edges of some of the rigid sheets. Where an interlayer has extended portions, these portions may include one or more reinforcing frames or inserts of aluminum, stainless steel, or other suitable material bonded within the interlayer in a plane substantially parallel to that of the viewing surfaces of the panel and extending inwardly beyond the edges of the glass. The interlayer extension and reinforcing frame serve to provide a somewhat resilient means to securely mount the panel to the aircraft body. The reinforcing frame or insert imparts increased shear resistance to the extended portion of the interlayer in a normal installation.
Several problems have arisen with existing aircraft panel glazings during service including separation of the plies, particularly at the edges of the glazing, and cold chipping. Thus, the coefficient of thermal expansion of the polyvinyl butyral plastic is approximately six times that of glass in the normal temperature range in which aircraft is used. When the laminate is exposed to extremely low temperatures, the difference in thermal expansion of the glass and plastic frequently causes delamination or chipping of the glass at the bonded surfaces. Delamination occurs upon failure of the glass to plastic adhesive bond and cold chipping occurs as a result of the glass to plastic bond being stronger than the cohesive bond between adjacent portions of a glass surface. Generally, cold chipping and/or delamination originates along the edge of the interior surface of the glass sheets where it is theorized that the stress concentration is greatest. This stress concentration and subsequent delamination and/or cold chipping is even more pronounced along the edge of a larger glass sheet when the glass sheets are of unequal area. In actual aircraft applications, the stress concentration is intensified due to the pressure differential imposed upon the mounting at high altitude flying.
Previous attempts to reduce the tendency for delamination and cold chipping with subsequent moisture ingress and bus bar failure have been rewarded with a moderate degree of success. One method previously employed involves the addition of an adhesive, e.g. Thiokol, having low temperature elasticity to the interior marginal surface areas of the glass plies adjacent to the interlayer. Another method is to insert a parting material in the interlayer at varying depths from the inboard surfaces of the glass plies. Such parting materials may comprise cellophane and/or pressure sensitive Mylar and Teflon tapes.
It has now been discovered that by providing a layer of polyisobutylene resin between the plastic interlayer and the glass sheet at the marginal portions of the glazing unit and in the area of the bus bar or electrode on the glass the possibility of electrode failure, delamination, and cold chipping in such units can be greatly minimized if not entirely eliminated. In other words, it has been found that this specific material may be employed as an improved chip retarder, parting medium and sealant for aircraft glazings.
It is accordingly an object of this invention to provide an improved laminated electrically conducting aircraft glazing.
More particularly, it is an object of the invention to eliminate electrode failure in an electrically heated aircraft glazing even under the most extreme temperature conditions.
Other objects and advantages of the invention will become more apparent during the course of the following description when taken in connection with the accompanying drawings.